Top stags sire more sons

November 30, 2006By John WhitfieldThis article courtesy of Nature News.

Male deer with better sperm invest in male offspring.

The fertility of male red deer helps to determine the sex of their offspring, say biologists in Spain. The best-quality sperm tends to father sons, while the less healthy sperm tends to lead to daughters.

This lets the top stags beget more of the same, says Monserrat Gomendio of the National Museum of Natural Sciences in Madrid, but stops low-quality males passing on their burden. "Daughters won't be handicapped by the poor ejaculate quality of their fathers," she points out.

It has been previously found that the mother can affect the sex ratio of her offspring: dominant females produce more sons, with subordinates favouring daughters. If a female red deer can produce large, healthy offspring, it makes sense that they should be sons, because males must fight for mates, whereas a female's success at attracting a partner is much less dependent on physical strength.

"To date, the research has been biased towards looking at females," says ecologist Steve Albon of the Macaulay Institute in Aberdeen, UK. "The conclusion that both males and females may contribute to variation in the sex ratio is very important."

Mating game

Gomendio and her colleagues inseminated captive female Iberian red deer with sperm taken from recently culled wild males. "If you arrive within hours of death, the ejaculate is the same as from a live animal," she says. "You can use the semen to produce live young."

Offspring of different males differed radically in their sex ratios, ranging from three-quarters sons to three-quarters daughters. The most fertile males those with the highest proportion of normal sperm cells had more male offspring, the researchers report in Science1. Overall, equal numbers of males and females were born.

Gomendio's team went to considerable lengths to try and remove differences in female biology and behaviour from the equation, making sure that the does were in similar physical condition and were inseminated at the same time in their respective ovulation cycles.

Picky parents

What's still a mystery is exactly how males and females manipulate the sex ratio of their offspring. One possibility is that females can prevent male embryos implanting, or selectively abort them as seems to happen during cold, wet winters, after which more females are born.

Males might be able to produce sperm biased towards either the X or Y chromosome. Or it might be that sperm differ in quality perhaps less fertile stags have mutations on their Y chromosomes that hamper the sperm carrying them. Gomendio's team now plans to test these ideas.

But whether red deer, and similar species, match the sexes of the offspring to their status and resources is still being debated, says Tim Coulson of Imperial College London. Some studies have found that animals seem to do this, whereas others have not. "I don't think we're at a point where we can say that all large mammals do this," he says.

Like deer, humans seem to produce more males in good times, and more girls in uncertain or difficult conditions. Everything from the child's parents living apart2, to the traumatic effect of the 11 September terrorist attacks3 has been proposed as increasing the ratio of baby girls to boys.